Fuel vapor treatment systems are known for reducing fuel vapor produced in a fuel tank from dissipating into the atmosphere. They are so constructed that fuel vapor in a fuel tank is introduced into a canister containing an adsorbent. When the internal combustion engine is operated, fuel vapor adsorbed by the adsorbent flows away from the adsorbent due to negative pressure in an intake pipe. The fuel vapor is discharged (i.e., purged) into the intake pipe of the internal combustion engine through a purge pipe. When the fuel vapor is purged, the adsorbing capability of the adsorbent is restored.
When fuel vapor is being purged, it is typically necessary to control the air-fuel ratio of air-fuel mixture introduced into the internal combustion engine to ensure the air-fuel ratio is approximately equal to a target air-fuel ratio (a theoretical air-fuel ratio in general cases). Consequently, there have been proposed technologies for implementing an air-fuel ratio sensor for measuring air-fuel ratio. The air-fuel ratio sensor is provided in the exhaust pipe of an internal combustion engine. Feedback control is carried out based on an amount of deviation between an air-fuel ratio measured by the air-fuel ratio sensor and a target air-fuel ratio. Injection quantity is thereby controlled so that the air-fuel ratio of air-fuel mixture introduced into the internal combustion engine is approximately equal to the target air-fuel ratio. (Refer to JP-A-7-269419, for example.)
The apparatus disclosed in JP-A-7-269419 is so constructed that an amount of deviation between the air-fuel ratio and the target air-fuel ratio is measured with an air-fuel ratio sensor. Based on the measured amount of deviation, a state of the fuel vapor concentration of air-fuel mixture containing fuel vapor purged from the canister is determined. Thus, fuel vapor concentration is a kind of fuel status. Based on the determined fuel vapor concentration (i.e., fuel status), injection quantity is controlled such that the air-fuel ratio becomes approximately equal to the target air-fuel ratio.
In the apparatus disclosed in JP-A-7-269419, air-fuel ratio is measured with an air-fuel ratio sensor, and deviation between the measured air-fuel ratio and target air-fuel ratio is fed back to determine an injection quantity. Thus, injection quantity cannot be determined unless purging is carried out.
When purging is started, therefore, it is typically required to select a relatively low purge rate so that air-fuel ratio does not substantially fluctuate and to gradually increase the purge rate. Furthermore, when purging is resumed after interruption, it is typically required to lower the initial purge rate and gradually increase it. For this reason, a problem arises. An amount of purge cannot be sufficiently increased.